core

## Creating Customized Datum Reference Frames

PC-DMIS supports the ability to create customized datum references frames (DRFs) for FCF tolerances (for Position, Profile with datums, Symmetry, Coaxiality, Concentricity, and Runout dimensions) as defined in section 4.22 of the Y14.5 2009 GD&T standard. The standard states that if using a customized DRF, then the datum features and modifiers listed therein must be followed by lowercase letters inside square brackets. These lowercase letters (x,y,z and u,v,w) represent the designated degrees of freedom to be constrained.

The six degrees of freedom in 3D space (x,y,z,u,v, and w)

To properly orient the X, Y, and Z axes, you must align the coordinate system to the datum feature or features before you create the FCF for the tolerance. PC-DMIS requires that the primary datum feature be leveled to Z+, and the secondary datum feature be rotated to X+. If a degree of translational freedom (x, y, or z) is turned off using the Customized DRF check box, then the datum shift along that axis no longer uses the datum feature as a starting point. If a degree of translational freedom is not constrained by any datum feature, then the origin of the current alignment is used as a reference for the datum shift table.

Mark the Customized DRF check box on the Feature Control Frame tab of the XactMeasure GD&T dialog box (Insert | Dimension) to enable the ability to select degrees of freedom to constrain from the Feature Control Frame editor. (If you do not mark this check box, PC-DMIS instead assigns the degrees of freedom based on the "Y14.5.1M-2009 Mathematical Definition of Dimensioning and Tolerancing Principles" standard.)

Next, in the DRF, after you have specified a datum (and, if appropriate, any modifiers), you can then access a list with the degrees of freedom you are able to constrain:

Feature Control Frame Editor Showing Available Degrees of Freedom to Constrain

These letters match the lowercase letters depicted in the diagram at the start of this topic.

Finally, mark the check box next to the degree or degrees of freedom you want to constrain, and then press TAB or click outside of the list. The FCF preview shows the customized datum reference frame within square brackets.

You can select any degree of freedom to constrain for a datum that has not already been specified for a datum of higher precedence. For example, if your secondary datum constrains the x degree of freedom, then x is not available to the tertiary datum.